Testbeds for Transition Metal Dichalcogenide Photonics: Efficacy of

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Testbeds for Transition Metal Dichalcogenide Photonics: Efficacy of Light Emission Enhancement in Monomer vs. Dimer Nanoscale Antennae Mohammad H. Tahersima, M. Danang Birowosuto, Zhizhen Ma, William C. Coley, Michael D. Valentin, Sahar Naghibi Alvillar, I-Hsi Lu, Yao Zhou, Ibrahim Sarpkaya, Aimee Martinez, Ingrid Liao, Brandon N. Davis, Joseph Martinez, Dominic Martinez-Ta, Alison Guan, Ariana E. Nguyen, Ke Liu, Cesare Soci, Evan Reed, Ludwig Bartels, and Volker J. Sorger ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00208 • Publication Date (Web): 14 Jun 2017 Downloaded from http://pubs.acs.org on June 19, 2017

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Testbeds for Transition Metal Dichalcogenide Photonics: Efficacy of Light Emission Enhancement in Monomer vs. Dimer Nanoscale Antennae Mohammad H. Tahersima1, M. Danang Birowosuto3, Zhizhen Ma1, William C. Coley2, Michael D. Valentin2, Sahar Naghibi Alvillar2, I-Hsi Lu2, Yao Zhou4, Ibrahim Sarpkaya1, Aimee Martinez2, Ingrid Liao2, Brandon N. Davis2, Joseph Martinez2, Dominic Martinez-Ta2, Alison Guan2, Ariana E. Nguyen2, Ke Liu1, Cesare Soci3, Evan Reed4, Ludwig Bartels2, Volker J. Sorger1

1 Department of Electrical and Computer Engineering, George Washington University, 800 22nd Street NW, Washington, DC 20052, USA 2 Chemistry and Materials Science & Engineering, University of California, Riverside, California 92521, USA 3 Division of Physics and Applied Physics, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Singapore 4 Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA KEYWORDS: Optical Antenna, Light Emitter, Plasmonic, Florescence, 2D materials, WS2

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ABSTRACT Monolayer transition metal dichalcogenides (TMDs) are materials with unique potential for photonic and optoelectronic applications. They offer well-defined tunable direct band gaps in a broad electromagnetic spectral range. The small optical path across them naturally limits the light-matter interactions of these two-dimensional (2-D) materials, due to their atomic thinness. Nanoscale plasmonic antennae offer substantial increase of field strength over very short distances, comparable to the native thickness of the TMD. For instance, it has been demonstrated that plasmonic dimer antennae generate hot-spot field enhancements by orders of magnitude when an emitter is positioned exactly over the middle of their gap. However, 2-D materials cannot be grown or easily transferred, to reside mid-gap of the metallic dimer cavity. Hence, it is not plausible to simply take the peak intensity as the emission enhancement factor. Here we show that the emission enhancement generated in a 2-D TMD film by a monomer antenna cavity rivals that of dimer cavities at a reduced lithographic effort. We rationalize this finding by showing that the emission enhancement in dimer antennae depends not on the peak of the field enhancement at the center of the cavity, but rather from the average field enhancement across a plane located beneath the optical cavity where the emitting 2-D film is present. We test multiple dimer and monomer antenna geometries and observe a representative 3-fold emission enhancement for both monomer and dimer cavities as compared to the intrinsic emission of chemical vapor deposition (CVD)-synthesized WS2 flakes. This finding suggests facile control and enhancement of the photoluminescence yield of 2-D materials based on engineering of lightmatter interactions that can serve as test-bed for their rapid and detailed optical characterization.


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TOC Graphics:

Nanoscale on-chip light-emitting structures are desired for a broad range of applications including displays, sensors, and optical interconnects. Transition-metal dichalcogenides (TMDs) are 2-D materials that show photoluminescence in the visible optical spectrum due to the emergence of a direct bandgap at the K point in the Brillouin zone. They also offer the potential for high quantum-yield emission and pure excitonic states [1-4]. TMDs are attractive materials for novel nanoscale optical emitters and optoelectronic devices [5-11]. The structural, chemical and electronically similar monolayer semiconducting TMD MoS2, MoSe2, WS2, and WSe2 provide light emission in the visible and near-infrared spectral regions (1.1 eV – 2.0 eV) [1-4]. The spatial confinement of carriers to a 3-atom thin physical plane and the weak dielectric screening in atomically-thin materials lead to high oscillator strengths and strong Coulombic interactions between the excited electron-hole pairs. This results in strong binding energies allowing for observation of excitons at room temperature [11-18]. In addition to neutral excitons, charged trions can also be excited in the presence of residual excess charge carriers. These quasi particles consist either of two electrons and one hole (A-), or one electron and two holes (A+). Therefore, electrostatic gating modifies the spectral weight of charge-neutral excitonic species in TMDs [14-17]. Moreover, given the large binding energy of the excitons, the


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formation of states consisting of two excitons (biexciton) is possible in TMDs, whose photoluminescence (PL) emission is red-shifted due to the additional binding energy [14, 18, 19]. A drawback of using TMD materials for photonic and optoelectronic applications is the low modal overlap with any externally-applied optical field because of the atomic thickness of the monolayer material and the fundamentally weak light-matter interaction of bosons with fermions. Different strategies to tailor light matter interaction and enhance both absorption and emission of these materials have been developed such as resonant (cavity, surface-plasmonresonance) and non-resonant (waveguide dispersion, metamaterials, index tuning) systems were utilized to achieve maximum light absorption and emission [20-35]. Plasmonic nanoantennae fall into the former category and they can synergistically a) increase the absorption cross-section thereby enhancing the pump efficiency, b) accelerate the internal emission rate via the Purcell effect through the nanoscale optical mode of the antenna, and c) improve emission out-coupling to free-space via impedance matching (transformer action). As such, optical antennae increase the excitation rate while simultaneously enhancing the local density of states (DOS) in the emission process, which modifies (here accelerates) the spontaneous emission rate known as the Purcell effect [25]. Hence, these optical antennae behave as electromagnetic cavities that strongly modify spontaneous emission of fluorescence in the spatial and spectral proximity. Plasmonic antennae are unique cavities; a good antenna has a low quality (Q)-factor and is, thus, an effective radiator. However, the light-matter interaction enhancement quantifier is proportional to the ratio of Q/V, where V is the optical mode volume. Given the possibility for sub-diffraction limited plasmonic optical mode volumes, the inherently low Q factors ensure a decent antenna function, while high Purcell factors of ~10’s to 100’s are obtainable [27,37,38]. Compared to photonic high-Q cavities, plasmonic antennae allow for simultaneous high


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absorption and PL emission, if quenching is prevented. Antenna-enhanced light-emitters have short radiative lifetimes and can have a deep subwavelength optical mode, thus opening the possibility of creating ultrafast, nanoscale emitters [26]. Recently, multiple TMD-plasmonic hybrid nanostructures have been investigated [27-35]. However, a detailed study probing the actual origin of light emission enhancement in these hybrid nanostructures have not yet been reported. A common claim in previous studies is that the gap-mode of the plasmonic dimer’s field enhancement enabling high Purcell factors. Plasmonic dimers, which are nanoscale structures consisting of two metallic nanoparticles separated by a small gap, support hybridized plasmon resonances because of the capacitive coupling between the plasmon modes of each nanoparticle. For a quantum emitter (e.g. =

𝐼𝑐𝑎𝑣 𝑆𝑐𝑎𝑣 𝐼0

⁄

𝑆0

(7)


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where 𝑆𝑐𝑎𝑣 is the area of a cavity, 𝑆0 is the area of the Gaussian excitation beam, 𝐼𝑐𝑎𝑣 is the PL intensity from the cavity, and 𝐼0 is the PL intensity from the as-grown WS2 flake; resulting in an < 𝐸𝐹𝑒𝑥𝑝 > of 50.9 and 39.7 fold. This evaluation leads to an overestimation of the local enhancement within the cavity from our experimental result, because per our simulation the electric field intensity and Purcell factor distribution directly under nanoantenna are significantly larger than the area around the antenna 𝑆𝑐𝑎𝑣 . Consequently, our findings point to the need for an evaluation of the field enhancement over a much larger area than 𝑆𝑐𝑎𝑣 in order to deduce realistic material properties from the cavity experiments. The shape of the PL spectrum in the cavity is conceptually similar to that of the control sample. However, we observe a spectral narrowing in FWHM value from 16.7 nm to 15.7, 13.5, and 14.6 nm respectively for the cases of the 75 nm radius monomer disc antenna, the 75 nm and 100 nm radii dimer antennae, respectively. We note that these effects are not sustained for the PL spectrum of the 200-nm radius dimer antenna for which we observe a red shift by 3.5 nm (relative to the intrinsic emission spectrum of as-grown WS2 flakes), and a band broadening to 21.8 nm. We attribute this observation to convolution of the PL emission spectrum of monolayer WS2 with the fundamental resonance of the 200-nm dimer cavity (Supporting Information). Supporting modeling indicates narrowing of the emission spectrum with the dot radius, so that for the cases of the 75 nm and 100 nm radii dimers the resonance condition affects narrowing and not broadening of the WS2 emission peak. This results in higher quality factor and sharper spectral response (Supporting Information), in line with a prediction based on Wheeler’s limit [50]. Our experimental findings highlights that the quality factor of nanoantennae increases as the dimensions of the disc decreases.


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In conclusion, we have demonstrated that optical nanoantennae can be used to control the emitting properties of monolayer TMDs. This control was achieved using two types of metallic cavities (monomer vs dimer) at four different sizes. These emission dynamics were also supported by numerical calculations. In particular, we have demonstrated the fluorescent enhancement of 2-D materials, and unlike quantum dots there is an areal average effect that has to be taken into account. We have also observed band narrowing of the emission response when resonance of the cavity corresponds to emission wavelength of monolayer WS2. Both monomer and dimer nanoantenna architecture are scalable to emission resonances of other members of the TMD family as well including MoTe2 which emits at a telecommunication wavelength in the near infrared. The demonstrated nano-antenna controlled emission from a monolayer WS2 flake could open a pathway to visible light sources based on lithographically fabricated nano-antennae supporting a variety of opto-electronic applications [51-56].

METHODS Growth method Single- and Multi-layer Tungsten Disulfide (WS2) was grown via ambient-pressure chemical vapor deposition (CVD) utilizing a tube furnace. The process is a variation of our previously published work on transition metal dichalcogenide materials [57-59]. The reagents were ammonia meta tungstate (AMT) and elemental sulfur. The process starts by spin-coating a 3.1mmol aqueous solution of AMT onto a SiO2/Si substrate. The resultant residue serves as the tungsten source for our CVD growth. We place a target substrate directly face-down onto this source substrate and insert this stack into the growth region at the center of the process tube in our tube furnace. An alumina boat containing elemental sulfur is placed inside the process tube


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far enough upstream of the furnace heating coil so that it just fully melts when the furnace center reaches the peak growth temperature of 850°C. The temperature ramp for WS2 growth commences after a nitrogen purge of the process tube for 15 minutes at 5 SCFH. After that we ramp the furnace to 500°C so as to decompose the AMT into tungsten trioxide releasing water and ammonia vapor. After 20 minutes the reaction is complete and we ramp up to the growth temperature of 850°C and remain there for 15 minutes. Subsequently, the furnace is allowed to cool naturally to 200°C before the process tube is opened to air and the target substrate retrieved. Fabrication method After WS2 films were deposited, a ~8 nm of aluminum oxide (Al2O3) was deposited onto the sample as a spacer between the emitter and the plasmonic antennae. Next, we fabricated monomer and dimer optical disk antennae, varying the diameters and gaps, onto the large area WS2 film using electron beam lithography. We then followed with gold deposition using electron beam evaporation. Finally the gold film is lifted-off using PG remover. Lithographical fabrication of the antennae allows for control over dimensions and position of particles, permitting us to tune the plasmonic resonance of each antenna element. The nanoantenna were arranged at a center-to-center distance of 4 µm to prevent interference of absorption cross section of individual antennae (Supporting Information). Raman and Photoluminescence Spectroscopy Raman and PL spectroscopy were performed using a custom-built spectrometer equipped with a 532-nm excitation laser and a CCD detector (Supporting Information). The nonlinear emission spectra were acquired in reflection geometry to a spectrometer including a 532-nm notch filter to reject the pump wavelengths. The spectrometer acquisition parameters were held constant and


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set to ensure high signal-to-noise ratio for the weakest signal. All measurements were performed in air at ∼25°C and atmospheric pressure. Reflectance Spectroscopy Reflectance measurements were performed using a 20W halogen light source by measuring the difference in reflected intensity from the WS2/SiO2 and bare silicon wafer substrate and normalizing this to the substrate-reflected intensity. For optical microscopy, we used white light and a 100x objective lens. FDTD calculation method Reflectance, in-plane and out-of-plane electric field intensities, Purcell factor, scattering, and absorption spectra were calculated. Optical constants of WS2 were obtained from Liu et al [60]. The refractive index of silica, silicon, alumina, and gold were used directly from Palik as well as Johnson and Christy [61, 62]. We used the Lumerical three-dimensional finite-difference time-domain (FDTD) solver for all Maxwell equation calculations. For scattering effect calculations, we employed a total field scattered field (TFSF) method. The plane wave was launched normally from the top of monomer or dimer antennae. Two (virtual) power-flow monitors (six detectors each) were positioned inside and outside the TFSF source, surrounding the antenna, to measure the absorption and scattering cross sections, in order. The power flow analysis calculates the net absorbed power and scattered power from the particle. For Purcell factor calculations, a dipole source was used (Supporting Information). In all other problems, a normal incident broadband plane wave was implemented. To ensure that scattered light does not return to the simulation region a Perfect Matching Layer (PML) was applied as boundary condition in all directions.


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For Purcell factor and Quantum efficiency calculations, we sampled the plane at the level of the 2-D TMD sheet with a dipole source. We calculated the radiated power by a set of (virtual) field power monitors as one transmission box surrounding only the dipole source and another transmission box far away from the source but surrounding both the emitter and cavity. The expression deployed in this method to evaluate the Purcell factor is Fp = Emitted-Power (f) / Source-Power (f), where f is the optical frequency. The Source-Power returns the power that the dipole would radiate in a homogeneous material. The Emitted-Power is the power transmitted out of a boxed area surrounding the dipole source due to the dispersive materials used in the cavity.

Supporting Information Available: [Description of near field intensity distribution, scattering efficiency, absorption loss map, and quality factor for all dimer antennae; additional Purcell factor results for a narrow gap dime antenna; nanoantenna fabrication steps and SEM images; PL measurement setup; DFT calculation results] The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.XXXXXXX.

ACKNOWLEDGMENT We thank the National Science Foundation and the Materials Genome Initiative for support under the award number NSF DMREF 14363300/1455050 and NSF EAPSI 1613966. V.S. is supported by Air Force Office of Scientific Research Young Investigator Program under grant FA9550-14-1-0215 and FA9550-14-1-0378.

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Testbeds for Transition Metal Dichalcogenide Photonics: Efficacy of

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